It’s a balmy summer evening. The sweltering heat and humidity of the day are subsiding, and the fireflies are beginning to twinkle in the corners of the yard. Ahhh. . .at last, time to relax on the back porch and watch the darkening sky fill up with stars. But, too soon, the peaceful revery is marred by an all too familiar buzzing sound near your ears, and a series of pinprick itches begin to emerge on your neck and arms and legs. Soothing solitude quickly transforms into your own personal rendition of the Mosquito Two Step. . .complete with flailing gestures and knee jerking, neck slapping choreography. They have won again; and as you reluctantly retreat indoors, you wonder why God added such pesky creatures to the Universe. Let’s explore the world of mosquitoes in this week’s Insight.
Mosquitoes have been around for 100 million years. During this time period, they have diversified into 3,000 species with adaptation to climates from the arctic to the equator. Approximately 170 of these species live in the United States. Worldwide, mosquito-borne diseases kill more people than any other single factor; these diseases include malaria, yellow fever, and dengue fever. In the United States, mosquitoes can spread several types of encephalitis and are capable of transmitting heart worms to dogs and cats. In addition to being disease vectors, mosquitoes can cause major disruptions, through their persistent biting, to occupational, recreational, and social activities.
Mosquitoes belong to the family of flies–possessing six delicate legs and two wings covered in scales. On their heads, they have a projecting proboscis which conceals and protects the long piercing and sucking mouthparts. Though their life cycle is complex, their life span is short. On average, a female mosquito will live 2–3 weeks, but wouldn’t you know that the male’s life is even shorter. The immature stage is totally aquatic while the adult is terrestrial. The adult female returns to a water habitat only for brief periods to lay each batch of eggs. With so many different species, the breeding habits, biting behavior, host preferences, and flight ranges vary tremendously. Many mosquito species clearly prefer other animals such as horses, frogs, turtles and especially birds to humans. Most remain within one mile of their breeding site, although there are salt marsh mosquitoes that will migrate 75–100 miles–at an average speed of 1 to 1.5 miles/hour. Can you imagine how the “kids” in the back seat would complain about THAT journey?!
Within their lifetime, both adult male and female mosquitoes will feed on nectar and plant fluids, but it is only the female that will seek a blood meal. Contrary to popular belief, mosquitoes do not suck blood for their own survival; rather, the blood meal serves as a protein source for egg development. The females are attracted to potential hosts through a combination of different stimuli emanating from the host which include: carbon dioxide and lactic acid production, body odors, air movement and body heat. Once she finds a host, the mosquito will probe the skin for a capillary and then inject a small amount of saliva which contains an anticoagulant to prevent clotting during her 5-millionths of a liter serving of blood. This saliva transmission is often the pathway for potential pathogens such as viruses to enter the host.
After her blood meal, the female will find a resting place to digest and develop her eggs before flying off to deposit an average of 100-300 eggs (in the form of a miniature raft) on the surface of standing water. Larval development generally takes up to two weeks and requires transition through four stages to produce an adult mosquito. Unfortunately for us, an adult female can produce up to 3000 offspring in her brief but remarkably fecund life. But remember, mosquito larvae are food for fish and other meat-eating water creatures, and the adults are food for bats, birds, dragonflies, and spiders. One reason many birds migrate north each year to breed is because of the plentiful supply of insects (including mosquitoes) that are available to feed their young.
Sensitivity to mosquito bites varies between individuals. Most people have only mild reactions with swelling, redness, and an itchy irritation at the puncture site. However, a smaller though significant percentage can have rather severe reactions to the saliva. Additionally, if the bites are scratched or traumatized in other ways, the irritated skin can become secondarily infected with bacteria.
So how do we remain Happy Campers when serving as a main course for such ferocious feeders? There are some general guidelines for protection, and some general misconceptions that need clarification. Premises control with the elimination of large breeding areas remains the most effective way to combat mosquitoes. In communities, eliminating swamps, sluggishly moving streams, and stagnant, open ditches can make a tremendous difference. On one’s own property, elimination of standing water is the key. This includes disposal of tin cans, old tires, buckets, unused plastic swimming pools and other containers that collect and hold water. Water at the base of flower pots or in pet dishes should not be allowed to accumulate for more than two days. Debris in rain gutters, standing water under faucets and air conditioners, and water in bird baths, ornamental pools, and standing water from irrigation systems can all create breeding grounds. There are biological larvicides available for ornamental pools that are intended to remain.
Backyard sprays and foggers are generally ineffective and can be environmentally harmful. They are effective for only 2 to 4 hours, and then the mosquitoes are back. Indiscriminate, these sprays kill every insect. . .including ladybugs, butterflys. . .even earthworms. And, in as little as 6 generations, or approximately 2 months, the surviving mosquitoes have usually developed an immunity to the pesticide. . .so you’ve created a super resistant strain of blood brother in the process! Citronella candles and smoking coils do repel mosquitoes, but only when they are actually IN the smoky plume. One study actually showed that mosquitoes who found a human’s shin covered in smoke flew around back and bit him on the unprotected calf! Finally, the ever-popular ultra-violet light bug zappers aren’t really very helpful either. They attract ALL night-flying insects and kill about 3,000 beneficial insects such as moths and butterflies each night–but only a handful of mosquitoes. A study conducted by Notre Dame University showed that people with zappers in their backyards actually got bitten on average 10% more than their non-zapper neighbors, because the zappers attracted mosquitoes but didn’t kill them. Additionally, the UV light emitted by zappers polarizes when it reflects off standing water, and the mosquitoes use the reflection to find new breeding spots where they can deposit their eggs. Finally, because large insects are attracted to zappers, when their bodies are exploded by the devices, the bug-fragments drift through the air, and can land decoratively on nearby people or deliciously in nearby food.
Topical insect repellents do work, and can be applied effectively to exposed areas of skin, but use, especially on young children, with discretion. Avon’s Skin-So-Soft seems to work as an effective repellent for some folks. Clothing with long sleeves and long pants can provide topical protection as well, though 85 degree plus temperatures and high humidity don’t often promote summer evening desire for such attire. Unfortunately, retreating indoors at twilight or at least behind screens, still seems to be one of the most effective protections against these predominately evening feeders. Though these biting remarks may not be welcomed news, I hope they’ll at least allow you to sit out a few sets of the Mosquito Two Step this summer.
Stephen L. Hines, M.D.
July 2001
